Today, microprocessor-based personal computers (PCs) find wide application in education, science, business and the home. As the use of personal computers becomes more widespread, the demand for faster and more flexible video features has also expanded. Consequently, computer manufacturers are diligently searching for ways to increase the performance and adaptability of video display systems while reducing the cost to the consumer.
In general, the internal architecture of the personal computer is organized such that the central processing unit (CPU) is housed on a printed circuit board which also contains system memory and supporting logic devices. This board is commonly referred to as a "motherboard". In the past, if users desired video graphics features, they necessarily had to purchase a separate video card which was designed to be plugged into a slot coupled to the motherboard across a connective bus interface. This card would contain dual-ported video random-access memories (VRAMs) which would be used to store the video display data later output to the display device (i.e., a monitor). The video card would have its video timing circuitry configured for a particular type of monitor; that is, the card could only be used with that type of monitor and no other. This past approach was typical of machines such as the original Macintosh II series computers, and is still in wide use today.
The use of a separate video card, however, has several important disadvantages. First of all, there is a sacrifice in speed performance since signals from the CPU must be transmitted across a slower communications bus to the separate video card. For example, in the Macintosh II family of computers, a "NUBUS" bus (NUBUS.TM. is a trademark of Texas Instruments Corporation) provides the connection between the CPU and the video card. This results in a significant loss in CPU bandwidth since the CPU cannot process information (e.g., dynamically update the video data) during the time it is transmitting a video word across the bus to the video card--a transfer which, in the case of a NUBUS, typically takes 5 CPU clock cycles plus about 700 nanoseconds (ns).
Secondly, the separate video card approach is very costly. Besides the cost of the extra board and connectors, the user must also sacrifice one of the available slots connected to the NUBUS. Moreover, VRAMs are about twice as expensive as the normal computer system RAM and a large amount of VRAM needs to be incorporated on the separate video card. VRAM memory is available only in certain sizes, which often do not match the requirements of a particular display. In many cases, a large portion of the memory space is simply "wasted" because the computer system cannot make effective use of it. In addition, a separate video card generates a considerable amount of extra heat due to the dissipation of power from its additional components. Heat generation is a major problem in computers today.
Perhaps a more fundamental limitation is that the user needs a different video card for each type of display or monitor that the computer is connected to. For example, a computer utilized to produce an image on a 15-inch portrait color monitor requires one type of video card, while one coupled to a 9-inch Black and White screen requires another. Thus, different monitors require matched video cards which ultimately reduce the flexibility afforded the user.
As will be seen, the present invention eliminates the need for a separate video card in a microprocessor-based computer system. It accomplishes this by the use of a video integrated circuit (IC) which may be incorporated directly onto the motherboard of the computer. The invented computer is characterized by one or more banks of system RAM which are shared by both the CPU and the video display circuitry. Because the video display uses the standard system RAM, which is completely accessible to the CPU, many different displays are supported without "wasting" any portion of the memory not used by the display. Hence, any portion of the system memory not being used by the video display circuitry may be used by the CPU to store instructions or perform calculations.
In a sense, the present invention is a form of direct memory access (DMA) in which the video IC reads information from the system RAM without any CPU intervention. The more usual way to implement DMA is to arbitrate the entire data bus--essentially disconnecting the CPU from the bus during a video display cycle. In contrast, according to the present invention the CPU can continue to work from its internal cache, external cache, ROM, I/O devices, NUBUS and certain banks of RAM, while the video IC is accessing video data from another bank of system RAM.
Further, because the present invention eliminates the need to communicate across a slower bus to a video card, system performance is ultimately enhanced.